1. Field of the Invention
This invention has particular application to the pickling of steel sheets, sheets that are distinct cut lengths as opposed to coils of strip. The thicknesses of the sheets may range from 0.033 inch up to and including 0.500 inch. (In the heavier thicknesses, usually over about 0.250 inch, the cut lengths may be referred to as plates.) Pickling is defined as chemical removal of surface oxides and may be distinguished from removal of grease or dirt, that is, from a normal cleaning operation. (Material that has been oiled or greased, however, can be pickled with this invention, particularly if acid recovery is not important.) The oxides are formed during rolling on a hot strip mill. Sometimes the strip is sheared on the hot strip mill to cut lengths, or it may be sheared to cut length as a separate operation after rolling on the hot strip mill.
The sheet pickler may also be used to perform cleaning operations such as removing stain from sheets. This may be necessary where the hot strip mill coil is pickled on a strip pickle line and sheared into sheets, but the rinsing of the strip was done improperly so that it did not remove the acid stain. This invention, therefore, may also be used to pickle material that was not completely pickled on the strip picklers. This sheet pickler may also be used to remove rust spots from sheared sheets which have been held in inventory too long and have rusted.
Scale removal or pickling requires the most exposure time in the acid and is the determining factor in designing line speed and acid tank length once the acid concentration ranges and temperature range for operation have been selected.
2. Description of the Prior Art:
A search of this invention was conducted among the United States patents classified in the United States Patent Office. No ascertion is made that the closest prior art was developed by this search although that was indeed the intention thereof. Such search developed the following United States Patents Nos.: Buckman 451,264; Sague 493,560; Marsh et al. 1,392,780; Peacock 1,488,553; McSetroidge 1,679,435; Theiss 2,259,277; Theiss 2,259,278; Birkin 3,042,995; Laine 3,087,505; Maust 3,398,022 and Marshall et al. 3,451,452.
The search developed patents are of some interest. U.S. Patent No. 451,264 discloses cleaning horizonally disposed plates before they are coated. Patent No. 493,560 is similar, referring specifically to a pickling bath and disclosing further that certain rolls and the like are located within the vat, although these apparently are not driven rolls. Continuous circulation of a pickling solution is shown in U.S. Pat. No. 1,392,780. Patent No. 1,488,553 appears to show positively driven rollers submerged along with the horizontal sheet being treated. Patent 1,679,435 speaks of the prior art method of pickling sheets by submersing them in a hot pickle or acid bath, apparently while they are vertically disposed. U.S. Patents Nos. 2,259,277 and 2,259,278 disclose a pickling tank including feed rollers which are positively driven. U.S. Pat. No. 3,042,995 discloses the coating of steel pickling rollers with a plastic material. U.S. Pat. No. 3,087,505 discloses pickling apparatus for handling strips wherein certain of the rolls are submerged in the pickling bath. And U.S. Pat. No. 3,451,452 discloses the use of polypropylene plastic in controlling corrosion when HCL is used for pickling.
Typically, in the prior art the sheets were batch pickled in sulfuric acid while vertically disposed, and then processed further through the scrubber line which wetted and rinsed the surface of the sheets prior to oiling and piling. It was not unusual for such an operation to require 15 men a turn to operate. Such prior condition presented a safety hazard in that men had to reposition the sheets from the horizontal to the vertical and then vice versa. Sulfuric acid fumes were ever present in the pickling room. The sheets often contained pickle pin digs and scratches.